In a wireless network, a wireless device may communicate with one or more radio network nodes to transmit and/or receive voice traffic, data traffic, control signals, and so on. Maintaining good signal quality between the wireless device and the radio network node may allow for good performance, such as high bitrate transmissions or robust control channel performance. However, it may be difficult to maintain good signal quality in complex radio environments. For example, interfering cells may create noise that interferes with the signal quality.
FIG. 1 illustrates the basic LTE physical resource as a time-frequency grid. The physical layer transmission in LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink. Thus, the basic LTE physical resource can be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one subcarrier during one OFDM symbol interval. For example, resource element 110 corresponds to one subcarrier during one OFDM symbol interval.
FIG. 2 illustrates a radio frame 210 of a downlink LTE transmission in the time domain. In the time domain, LTE downlink transmissions are organized into radio frames, such as radio frame 210, of 10 ms. Each radio frame 210 includes ten equally-sized subframes of 1 ms, illustrated in FIG. 2 as subframes #0 through #9. A subframe, such as subframe #0, is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks (RB), where an RB corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive RBs represent an RB pair, and correspond to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where a network node base station (e.g., an eNodeB (eNB)) transmits downlink assignments/uplink grants to certain wireless devices (e.g., user equipment (UE)) via the physical downlink control channel (PDCCH), or the enhanced PDCCH (ePDCCH) introduced in LTE Rel. 11. In certain embodiments, the network node and wireless device may be network node 610 and wireless device 605 described below in relation to FIG. 6. In LTE downlink, data is carried by the physical downlink shared channel (PDSCH), and in the uplink the corresponding link is referred to as the physical uplink shared channel (PUSCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and span (more or less) the whole system bandwidth, whereas ePDCCH is mapped on RBs within the same resource region as used for PDSCH. Hence, ePDCCHs are multiplexed in the frequency domain with the PDSCH, and may be allocated over the entire subframe. A UE that has decoded an assignment carried by a PDCCH, or ePDCCH, knows which resource elements in the subframe contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows which time/frequency resources it should transmit upon.
Demodulation of sent data requires estimation of the radio channel, which is done by using transmitted reference symbols (RS), i.e. symbols known by the receiver. In LTE, cell specific reference symbols (CRS) are transmitted in all downlink subframes and in addition to assist downlink channel estimation they are also used for mobility measurements performed by the UEs. LTE also supports UE specific RS, i.e. demodulation reference signals (DMRS), for assisting channel estimation for demodulation purposes.
FIG. 3 illustrates how the mapping of PDCCH and PDSCH and CRS can be done on resource elements within a downlink subframe 310. Downlink subframe 310 includes control region 320 and data region 330. In FIG. 3, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data carried by PDSCH could start already at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to DMRS, which means that each UE has reference signals of its own placed in the data region 330 of FIG. 3 as part of PDSCH. In LTE, subframes can be configured as MBSFN subframes, which implies that CRSs are only present in the PDCCH control region.
The length of the PDCCH control region 320, which can vary on subframe basis, is conveyed in the physical control format indicator channel (PCFICH). The PCFICH is transmitted within control region 320, at locations known by UEs. After a UE has decoded the PCFICH, it thus knows the size of control region 320 and in which OFDM symbol the data transmission starts. Also transmitted in control region 320 is the physical hybrid-ARQ indicator channel (PHICH). This channel carries ACK/NACK responses to a UE to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
In LTE, DMRSs are introduced in order to allow for demodulation of data based on UE specific RSs. These RSs are placed in data region 330, and are described in more detail in FIG. 4.
FIG. 4 illustrates an example of UE-specific reference symbols. R7 and R9 represent the DMRSs corresponding to antenna port 7 and 9, respectively. In addition, antenna port 8 and 10 can be obtained by applying an orthogonal cover as (1-1) over adjacent pairs of R7 and R9, respectively
As previously indicated, CRS and DMRS are not the only reference symbols available in LTE. As of LTE Rel. 10, a new RS concept was introduced with separate UE specific RS for demodulation of PDSCH and RS for measuring the channel for the purpose of channel state information (CSI) feedback from the UE. The latter is referred to as CSI-RS. CSI-RS are not transmitted in every subframe, and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe, according to an RRC configured periodicity parameter and an RRC configured subframe offset.
FIG. 5 illustrates the resource elements within a resource block pair that may potentially be occupied by UE specific RS and CSI-RS. The CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As illustrated in FIG. 5, many different CSI-RS patterns are available. As one example, there may be two CSI-RS antenna ports as illustrated in CSI-RS pattern 510. In certain embodiments, where there are two CSI-RS antenna ports, there are twenty different patterns within a subframe. As another example, there may be four CSI-RS antenna ports, as illustrated in CSI-RS pattern 520, or eight CSI-RS antenna ports, as illustrated in CSI-RS pattern 530. The corresponding number of patterns is 10 and 5 for four and eight CSI-RS antenna ports, respectively. For TDD, some additional CSI-RS patterns are available.
Subsequently, the term CSI-RS resource may be mentioned. In such a case, a resource corresponds to a particular pattern present in a particular subframe. Thus, two different patterns in the same subframe, or the same CSI-RS pattern but in different subframes, in both cases constitute two separate CSI-RS resources.
The CSI-RS patterns may also correspond to so-called zero-power CSI-RS, also referred to as muted REs. Zero-power CSI-RS corresponds to a CSI-RS pattern whose REs are silent, i.e., there is no transmitted signal on those REs. Such silent patterns are configured with a resolution corresponding to the 4 antenna port CSI-RS patterns 520 illustrated in FIG. 5. Hence, the smallest unit to silence corresponds to four REs.
Conveying indications of physical layer resource allocation is one of the major functions of PDCCH (and now ePDCCH). In each subframe this control channel indicates the PDSCH resource allocations. Several resource allocation types are defined in LTE. As one example, the resource allocation type may be resource type allocation 0, where a bitmap indicates the Resource Block Group (RBG) which are allocated to the scheduled UE, where the RBG is a set of consecutive PRBs whose size depend on the system bandwidth. For example, the number of RBG when NRBDL available PRBs are present is given by P obtained asNRBG=ceil(NRBDL/P)
As another example, the resource type allocation may be resource allocation type 1, where individual PRBs can be addressed, but only within a subset of PRBs available within the RBG. This resource type allocation allows spreading in frequency domain to be achieved, which exploits frequency diversity. As yet another example, the resource allocation may be resource allocation type 2, where the resource allocation information indicates a contiguous set of PRB, using either localized or distributed mapping. Resource allocations can be localized, meaning that a PRB in the first half of a subframe is paired with the PRB at the same frequency in the second half of the subframe, or distributed meaning that the two physical RBs in a PRB pair are separated in frequency domain. This achieves better frequency diversity when a small amount of data has to be transmitted. Under resource allocation type 2, the PRB allocation may vary from a single PRB up to a maximum number of PRBs spanning the entire system bandwidth.